Re: [REGRESSION] Select hang with zero sized UDP packets

[Eric Dumazet]

On Tue, 2016-08-23 at 11:25 -0700, David Miller wrote:
> From: Laura Abbott <labb...@redhat.com>
> Date: Tue, 23 Aug 2016 10:53:26 -0700
> 
> > Fedora received a report[1] of a unit test failing on Ruby when using
> > the
> > 4.7 kernel. This was a test to send a zero sized UDP packet. With the
> > 4.7 kernel, the test now timing out on a select instead of completing.
> > The reduced ruby test is
> > 
> >   def test_udp_recvfrom_nonblock
> >     u1 = UDPSocket.new
> >     u2 = UDPSocket.new
> >     u1.bind("127.0.0.1", 0)
> >     u2.send("", 0, u1.getsockname)
> >     IO.select [u1]  # test gets stuck here
> >   ensure
> >     u1.close if u1
> >     u2.close if u2
> >   end
> 
> Well, if there is no data, should select really wake up?
> 
> I think it's valid not to.
There are skb in receive queue, with skb->len = 0

This looks like a bug in first_packet_length() or poll logic.

Definitely something we can fix.

Maybe with :

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index e61f7cd65d08..380c05a84041 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1184,11 +1184,11 @@ out:
  *     Drops all bad checksum frames, until a valid one is found.
  *     Returns the length of found skb, or 0 if none is found.
  */
-static unsigned int first_packet_length(struct sock *sk)
+static int first_packet_length(struct sock *sk)
 {
        struct sk_buff_head list_kill, *rcvq = &sk->sk_receive_queue;
        struct sk_buff *skb;
-       unsigned int res;
+       int res;
 
        __skb_queue_head_init(&list_kill);
 
@@ -1203,7 +1203,7 @@ static unsigned int first_packet_length(struct sock *sk)
                __skb_unlink(skb, rcvq);
                __skb_queue_tail(&list_kill, skb);
        }
-       res = skb ? skb->len : 0;
+       res = skb ? skb->len : -1;
        spin_unlock_bh(&rcvq->lock);
 
        if (!skb_queue_empty(&list_kill)) {
@@ -1232,7 +1232,7 @@ int udp_ioctl(struct sock *sk, int cmd, unsigned long arg)
 
        case SIOCINQ:
        {
-               unsigned int amount = first_packet_length(sk);
+               int amount = max(0, first_packet_length(sk));
 
                return put_user(amount, (int __user *)arg);
        }
@@ -2184,7 +2184,7 @@ unsigned int udp_poll(struct file *file, struct socket 
*sock, poll_table *wait)
 
        /* Check for false positives due to checksum errors */
        if ((mask & POLLRDNORM) && !(file->f_flags & O_NONBLOCK) &&
-           !(sk->sk_shutdown & RCV_SHUTDOWN) && !first_packet_length(sk))
+           !(sk->sk_shutdown & RCV_SHUTDOWN) && first_packet_length(sk) == -1)
                mask &= ~(POLLIN | POLLRDNORM);
 
        return mask;